Could global warming increase wind shear over the Atlantic, potentially leading to a decrease in the frequency of Atlantic hurricanes? There is a growing consensus among hurricane scientists that this is indeed quite possible. Two recent studies, by Zhao et al. (2009), "Simulations of Global Hurricane Climatology, Interannual Variability, and Response to Global Warming Using a 50-km Resolution GCM", and by Knutson et al. (2008), "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", found that global warming might increase wind shear over the Atlantic by the end of the century, resulting in a decrease in the number of Atlantic hurricanes. For example, the second study took 18 relatively coarse (>60 km grid size) models used to formulate the 2007 IPCC climate report, and "downscaled" them using a higher-resolution (18 km grid size) model called ZETAC that was able to successfully simulate the frequencies of hurricanes over the past 50 years. When the 18 km ZETAC model was driven using the climate conditions we expect in 2100, as output by the 18 IPCC models, the authors found that a reduction of Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century resulted. An important reason that their model predicted a decrease in the frequency of Atlantic hurricanes was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

Figure 1. Top: predicted change by 2100 in wind shear (in meters per second per degree C of warming--multiply by two to get mph) as predicted by summing the predictions of 18 climate models. Bottom: The number of models that predict the effect shown in the top image. The dots show the locations where tropical storms formed between 1981-2005. The box indicates a region of frequent hurricane formation where wind shear is not predicted to change much. Image credit: Geophysical Research Letters, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", by Vecchi and Soden, 2007.

Since the Knutson et al. study using the 18 km resolution ZETAC model was not detailed enough to look at what might happen to major Category 3 and stronger hurricanes, a new study using a higher resolution model was needed. This was done by a team of modelers led by Dr. Morris Bender of NOAA's GFDL laboratory, who published their results in Science in February. The authors used the GFDL hurricane model--the model that has been our best-performing operation hurricane track forecasting model over the past five years--to perform their study. The GFDL hurricane model runs at a resolution of 9 km, which is detailed enough to make accurate simulations of major hurricanes. The researchers did a double downscaling study, where they first took the forecast atmospheric and oceanic conditions at generated by the coarse (>60 km grid) IPCC models, used these data to initialize the finer resolution 18 km ZETAC model, then used the output from the ZETAC model to initialize the high-resolution GFDL hurricane model. The final results of this "double downscaling" study suggest that although the total number of hurricanes is expected to decrease by the end of the century, we should expect an increase of 81% in the number of Category 4 and 5 storms in the Atlantic. This trend should not be clearly detectable until about 60 years from now, given a scenario in which CO2 doubles by 2100. The authors say that their model predicts that there should already have been a 20% increase in the number of Category 4 and 5 storms since the 1940s, given the approximate 0.5°C warming of the tropical Atlantic during that period. This trend is too small to be detectable, given the high natural variability and the difficulty we've had accurately measuring the exact strength of intense hurricanes before the 1980s.The region of the Atlantic expected to see the greatest increase in Category 4 and 5 storms by the year 2100 is over the Bahama Islands (Figure 2), since wind shear is not expected to increase in this region, and sea surface temperatures and atmospheric instability are expected to increase there.

The net effect of a decrease in total number of hurricanes but an increase in the strongest hurricanes should cause an increase in U.S. hurricane damages of about 30% by the end of the century, the authors compute, assuming that hurricane damages behave as they did during the past century. Over the past century, Category 4 and 5 hurricanes made up only 6% of all U.S. landfalls, but accounted for 48% of all U.S. damage (if normalized to account for increases in U.S. population and wealth, Pielke et al., 2008.)

Figure 2. Expected change in Atlantic Category 4 and 5 hurricanes per decade expected by the year 2100, according to the Science paper by Bender et al. (2010).

CommentaryThese results seem reasonable, since the models in question have been successfully been able to simulate the behavior of hurricanes over the past 50 years. However, the uncertainties are high and lot more research needs to be done before we can be confident of the results. Not all of the IPCC models predict an increase in wind shear over the tropical Atlantic by 2100, so the increase in Category 4 and 5 hurricanes could be much greater. Also, the GFDL model was observed to under-predict the strength of intense hurricanes in the current climate, so it may not be creating enough Category 4 and 5 hurricanes in the future climate of 2100. On the other hand, IPCC models such as the UKMO-HadCM3 predict a very large increase in wind shear, leading to a drastic reduction in all hurricanes in the Atlantic by 2100, including Category 4 and 5 storms. So Category 4 and 5 hurricane frequency could easily be much greater or much less than the 81% increase by 2100 found by Bender et al.

The estimates of a 30% increase in hurricane damages by 2100 may be considerably too low, since this estimate assumes that sea level rise will continue at the same pace as was observed in the 20th century. Sea level rise has accelerated since the 1990s, and it is likely that this century we will see much more than than the 7 inches of global sea level rise that was observed last century. Higher sea level rise rates will sharply increase the damages due to storm surge, which account for a large amount of the damage from intense Category 4 and 5 hurricanes.

Keep in mind that while a 30% in hurricane damage by the end of the century is significant, this will not be the main reason hurricane damages will increase this century. Hurricane damages are currently doubling every ten years, according to Pielke et al., 2008. This is primarily due to the increasing population along the coast and increased wealth of the population. The authors theorize that the Great Miami Hurricane of 1926, a Category 4 monster that made a direct hit on Miami Beach, would have caused about $150 billion in damage had it hit in 2005. By 2015, the authors expect the same hurricane would do $300 billion in damage. This number would increase to $600 billion by 2025 (though I think it is likely that the recent recession may delay this damage total a few years into the future.) It is essential that we limit coastal development in vulnerable coastal areas, particularly along barrier islands, to reduce some of the astronomical price tags hurricanes are going to be causing. Adoption and enforcement of strict building standards is also a must.

The authors of the GFDL hurricane model study have put together a nice web page with links to the paper and some detailed non-technical explanations of the paper.

Quoting Bordonaro:I am no expert, highly unlikely! If it was a 6.5 Mw, yes. Not much real estate moves or fractures in a 3.5 Mw quake.

Hey Bordo, let me put it another way. I wouldnt want a 0.1 magnitude earthquake within 100 miles from me inside one or any of those coal mines. And a 6.5 would do a lot more than collapse a coalmine.:) they are saying it was an explosion that killed the miners in West Virginia.

from a person who has never step foot in louisiana, mississippi or alabama and after all these years i still find it hard to watch images of katrina.

I have been to NOLA many times, my brother is a katrina victim who relocated from Slidell, he had 8 ft of water and mud in his house. But i never went to the 9th ward, only the touristy parts of NOLA, and I have not been in that area post Katrina. Those pictures and documentaries are horrifying tho, and the injustice of treatment people received is horrible. But I have frequently gone along hwy 90 in Gulfport and I traveled there a whole year after camille and I was shocked. a whole year later, big boats miles inland, the kmart blown away to nothing but a clean slab and tangled knotted shopping carts looked like they were made of pipe cleaners, all the giant old oaks. It was shocking. The beach there has never looked like it was pre camille.

Thanks for the laughs on my behalf. I can't believe some lady on here gave me that name. Maybe she was in a bad mood today!

i was just playing haha. I meant no animosity. I just think your forecasts are always so portentious and serious :) Just keep on, don't let my teasing get to you. i think jeffstradamas is a cool name! but i like jeff cayce too!

This whole Mars terraforming thing has me confused. I thought that scientists were supposed to be smart. If Mars lost it's atmosphere because it does not have an iron core and magnetic fields to protect it from solar wind like Earth does, what is going to keep the new atmosphere from blowing away as you make it??? Duh.

Above ground, Mars is mostly a bone-chilling desert pocked with craters. Hundreds of miles below, however, a molten sea of iron, nickel and sulfur churns. And new research suggests the gooey core will eventually solidify—either from the outside-in, forming an iron-nickel core, or from the inside out, forming a core of a fool's-gold-like minerals.

Andrew Stewart, a planetary geochemist at the Swiss Federal Institute of Technology, said Mars' cooling core might restore magnetism to the red planet. "If liquid metal moves around a solid core, it could create a natural dynamo like the one found in Earth's core," said Stewart, who co-authored the study detailed in today's online edition of the journal Science.

Why did Mars lose its atmosphere? That's the real mystery. A leading theory proposed by physicist Mario Acuna of the Goddard Space Flight Center conjectures that the solar wind gradually blows it away.

The Sun blasts Mars with charged particles that erode its atmosphere like a desert wind scours the land. Earth's magnetic field shields us from the solar wind, and protects our atmosphere. Unfortunately, Mars lost its shield. (See Figure 3)

Four billion years ago, Mars had a molten core whose electric currents generated a magnetic field that swathed the planet (like ours does today). We know this is true because we have found magnetic Martian rocks, dating older than 4 billion years. (See Figure 4)

Then, the planet lost its magnetic field. We sent instruments to Mars (onboard the Mars Global Surveyor satellite) to measure the magnetic field, and discovered it has no overall magnetic field %u2014 no magnetism emanating from its core.

So, with no magnetic shield to deflect the Sun's particles, the solar wind gradually blew the Martian atmosphere into space. Subsequently, Mars lost 70 to 90% of its water %u2014 blasted into space and the rest tied up in rocks.

THUNDERSTORM INTENSITIES WILL GRADUALLY INCREASE THIS AFTERNOON AS CONVECTION DEVELOPS EWD THROUGH THE MID-MS AND OH VALLEYS. MAIN THREAT WILL BE ISOLATED DAMAGING WIND AND POSSIBLY HAIL. HOWEVER...AN ISOLATED TORNADO OR TWO CANNOT BE RULED OUT. TRENDS ARE BEING MONITORED FOR A POSSIBLE WW.

A COLD FRONT EXTENDS FROM A SURFACE LOW IN WRN IL SSWWD THROUGH SERN MO...NRN AR AND ERN TX. WARM FRONT STRETCHES EWD FROM THE LOW THROUGH NRN IL...NWRN IND AND SRN LOWER MI. SATELLITE TRENDS INDICATE A FEW BREAKS DEVELOPING IN THE PRE-FRONTAL MULTI-LAYER CLOUD DECK CONTRIBUTING TO DESTABILIZATION OF THE BOUNDARY LAYER...AND STORMS HAVE RECENTLY INCREASED NEAR THE MO/IL BORDER. MLCAPE WILL LIKELY REMAIN AOB 1000 J/KG DUE TO MODEST LAPSE RATES AND LOW-LEVEL MOISTURE WITH DEWPOINTS AROUND 60. STORMS WILL CONTINUE DEVELOPING ALONG THE FRONT AS WELL AS WITHIN THE WEAKLY CAPPED WARM SECTOR THIS AFTERNOON. SWLY BULK SHEAR OF 40-50 KT WILL PERSIST DOWNSTREAM FROM EWD ADVANCING SHORTWAVE TROUGH. HOWEVER...WIND PROFILES ARE FORECAST TO REMAIN SOMEWHAT UNIDIRECTIONAL WITH 0-1 KM STORM RELATIVE HELICITY AROUND 150 M2/S2. SEVERE THREAT WILL GRADUALLY INCREASE AS THE BOUNDARY LAYER DESTABILIZES WITH BOTH DISCRETE SUPERCELLS AND LINE SEGMENTS LIKELY.

This whole Mars terraforming thing has me confused. I thought that scientists were supposed to be smart. If Mars lost it's atmosphere because it does not have an iron core and magnetic fields to protect it from solar wind like Earth does, what is going to keep the new atmosphere from blowing away as you make it??? Duh.

Good point. I have not read extensively on the subject for a long time. It would surprise me to find that Mars does not have an iron core. After all the reason it is the red planet is because of all the oxidized iron deposits on the surface. I will have to take some time and glean over the latest findings.That is if we still have power tonight.

Quoting hydrus: I still have a magazine from the late 80,s on the subject. It is interesting.

This whole Mars terraforming thing has me confused. I thought that scientists were supposed to be smart. If Mars lost it's atmosphere because it does not have an iron core and magnetic fields to protect it from solar wind like Earth does, what is going to keep the new atmosphere from blowing away as you make it??? Duh.

Change in SST Anomalies from February 16th to March 15th shows that some anomalous warming occurred in the MDR but substantial anomalous cooling in the GOM. There was also mostly slight anomalous cooling in the Nino regions of the Pacific.

IRI April SST forecast for July-August-September SSTs shows continued warmth in the tropical Atlantic and Caribbean, as well as a weak-moderate La Nina developing in the Pacific. The associated precipitation forecast reflects this with above-normal precipitation over the Caribbean. Also interesting is it's forecast for a much wetter-than-normal Sahel region of Africa.

NASA's new model forecast is also showing a negative NAO regime for the MJJ period, as well as strong 200mb ridging over the tropical Atlantic. These are both favorable patterns to set up the main part of the season. Notice the overlap between low sea-level pressures and high 200mb heights over the tropics south of 25N. That overlap is the area of most favorable conditions.

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS MUCH OF THE LOWER/MID MS AND LOWER OH VALLEYS...

16Z SURFACE ANALYSIS SHOWS A LOW ALONG THE MO/IL BORDER AND A WARM FRONT EXTENDING NORTHEASTWARD INTO IL/IND AND NORTHWEST OH. A COLD FRONT EXTENDS SOUTHWARD FROM THE LOW INTO NORTHWEST AR/SOUTHEAST OK AND EASTERN TX. BOTH OF THESE BOUNDARIES WILL TEND TO FOCUS STRONG/SEVERE STORMS LATER TODAY.

...IL/IND... VISIBLE SATELLITE IMAGERY SHOWS BREAKS IN THE CLOUDS ACROSS CENTRAL IL IN VICINITY OF THE WARM FRONT. LOW LEVEL MOISTURE IS ALSO INCREASING FROM THE SOUTH...WHICH SHOULD LEAD TO AFTERNOON MLCAPE VALUES OF 500-1000 J/KG IN THE REGION. SCATTERED THUNDERSTORMS ARE EXPECTED TO FORM AHEAD OF THE SURFACE LOW...BOTH ALONG THE WARM FRONT AND IN THE WARM SECTOR. FORECAST SOUNDINGS SHOW STRONG EFFECTIVE SHEAR AND SUFFICIENT CAPE FOR SUPERCELL STORMS. MESOSCALE MODELS ALSO INDICATE THIS REGION IS MOST LIKELY TO SEE DISCRETE INITIATION. LOW LEVEL VERTICAL SHEAR IS MODEST BUT SUFFICIENT FOR A RISK OF ISOLATED TORNADOES...ALONG WITH HAIL AND DAMAGING WINDS AS STORMS TRACK EASTWARD INTO IND.

...AR/MS/TN INTO OH VALLEY... SURFACE COLD FRONT OVER AR/LA/TX WILL LIKELY PROVIDE THE LOW LEVEL LIFT FOR A LINE OF SEVERE STORMS LATER TODAY. THIS ACTIVITY WILL SPREAD NORTHEASTWARD INTO PARTS OF MS/TN/KY THROUGH THE EVENING HOURS. A WEAK CAP COULD CONTRIBUTE TO A RISK OF DISCRETE SUPERCELLS EARLY IN THE EVENT...BUT LINEAR STRUCTURES WITH EMBEDDED BOW/LEWPS ARE EXPECTED THIS EVENING. DAMAGING WINDS AND HAIL APPEAR TO BE THE MAIN THREATS...ALTHOUGH ISOLATED TORNADOES CANNOT BE RULED OUT.

..HART/HURLBUT.. 04/07/2010

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